Question for rocket scientists

SecondCor521

Give me a museum and I'll fill it. (Picasso) Give me a forum ...
Joined
Jun 11, 2006
Messages
8,294
Location
Boise
I occasionally watch things about SpaceX and travel to Mars.

One topic that came up was how long it takes to get from Earth to Mars, with the current SpaceX answer with Starship being about six months one way, and only during a window of about six months out of every twenty four. I think the idea behind this approach is to launch in an elliptical orbit outward from Earth towards Mars, where the far end of the ellipse is Mars (or the orbit around Mars).

An idea occurred to me that there might be a faster way to get there. Here's my idea:

Instead of launching on a direct elliptical path, use a modified version of the slingshot idea that we used on Voyager 1 and 2. Launch from the Earth, slingshot around the trailing edge of the Moon towards Venus, then around the trailing edge of Venus out to Mars orbital insertion.

This would require Venus to be somewhat "behind" Earth in it's orbit and it probably works best if Mars is also behind Earth but ahead of Venus, with the Moon maybe ahead of and on the Solar side of it's orbit around the Earth. This may be a rare occurrence, but it seems like it should happen at least sometimes. If you look at The Planets Today : A live view of the solar system with today's date (10/7/24) and Earth and Venus are where they currently are, and Mars is where Ceres currently is and the Moon is where it was on about 9/28/2024, that's the arrangement I'm thinking could work.

It could be faster because both (a) slingshots can speed up your velocity, and (b) the overall distance could be shorter because for the Venus to Mars section you could be going "straight out" from the Sun rather than on the "elliptical orbit catchup" path from Earth out to Mars in the same direction (counterclockwise on the above website). Using slingshots to gain velocity and using a shorter path also could reduce the amount of fuel required, which might enable a somewhat larger payload.

A second, similar idea would be to skip Venus entirely and slingshot around the Moon and back out to Mars. For this similar idea, if the Moon is almost new (say 10/1/2024 location) and Mars is slightly behind the Earth, you could slingshot around the trailing edge of the Moon, then around the trailing edge of Earth, directly out towards Mars. This is one less slingshot, but requires one less celestial body to be in the right place and is arguably even more of a straight line.

Now of course, Elon Musk and all the SpaceX engineers are smart folks, so I'm 99.999% sure that my idea doesn't work for some reason, or it does work but it's suboptimal for some reason. If that's the case, I'd be interested to understand the reason why it isn't a good idea.

One problem is that the planets might not be in this arrangement often. The Moon seems not to be that big of an issue because it moves, relatively speaking, so much faster than Venus and Mars, so it should pass through an opportune location frequently enough.

Another possible thing that could be wrong with these ideas is the accuracy of the launch and slingshot might be beyond our reach. But it seems like we should be able to adjust the trajectories like we did several times on Apollo 13, so this latter issue seems conquerable.

Finally, maybe the slingshots around the Moon and Venus don't add much velocity because they're not particularly massive. But they are pretty massive compared to a Starship rocket, so it seems like that shouldn't be the issue.

There's always the 0.001% chance that I've thought of something that nobody else has. I do have a US utility patent to my name, so it's not the craziest thing that could occur.

I've just never heard of this idea even being mentioned, so I thought I'd throw it out there.
 
Have you considered how the ship is going to slow down so it can arrive at Mars with a velocity to orbit and not just blow by? As I understand it, the trip can accelerate only as much as it can decelerate at the far end.

Good question. A couple of comments and ideas:

With my idea you might not need as much velocity as SpaceX's current plan, because you're not traveling as far. You could choose a hopefully suitable combination of lower velocity but shorter distance that would both get you there faster and have less of a deceleration problem at the far end.

You can always turn around and do an orbital insertion burn. We did this on the Apollo moon missions to enter Moon orbit. An OI burn in the direction of travel reduces velocity. Of course this requires rocket fuel.

You can possibly use Mars' atmosphere and/or parachutes as a brake. Although Mars' atmosphere is thin compared to the Earth and parachutes are heavy.

Entering Mars' orbit on the trailing edge would help I think, but I think that's part of SpaceX's current plan. Although their plan is approaching at a different angle so the relative velocity could be "easier". So not really an improvement there.
 
Would your slingshot around Venus bring the spaceship too close to the Sun ??

Venus doesn't have a whole lot of gravitational pull, and the ship might be overwhelmed by the massive gravity of the Sun.

I'm sure the SpaceX engineers have thought this all out.
I'm not at all sure what Elon Musk has been thinking about.
 
My guess is slingshots will make the average speed faster but that advantage would be lost due to the mileage added to swing past Venus. It also means not only would Earth and Mars need to be at an advantageous relative position, but also Venus, which greatly reduces the number of viable launch windows.
 
Would your slingshot around Venus bring the spaceship too close to the Sun ??

Not really I don't think. If you're thinking about heat, the reason Venus is hot is because it absorbs a lot of heat which then gets trapped by the atmosphere. SpaceX's space ships can (and I think do) have reflective surfaces as well as no atmosphere.

Venus doesn't have a whole lot of gravitational pull, and the ship might be overwhelmed by the massive gravity of the Sun.

Gravity always pulls with a force proportional to the mass of the two objects and inversely to the square of the distance. There mathematically has to be some path (relatively close to Venus) where the closer distance to Venus and the further distance from the Sun equals out in the appropriate way. Whether we can accurately hit that spot and the margin of error is something I don't know.

I'm sure the SpaceX engineers have thought this all out.
I'm not at all sure what Elon Musk has been thinking about.

I'm 99.999% sure they have.

My guess is slingshots will make the average speed faster but that advantage would be lost due to the mileage added to swing past Venus. It also means not only would Earth and Mars need to be at an advantageous relative position, but also Venus, which greatly reduces the number of viable launch windows.

My point, that I probably didn't make very clear, is that the current plan seems to be to launch from Earth outward and in the same orbital direction as Mars, which requires the space ship to have a relatively high orbital velocity to "catch up" to Mars.

My idea is to use Venus to go "backwards" and intercept Mars, so Mars would be "closing towards" the space ship rather than "running away" from it. See attached poorly drawn PNG. All planets shown rotate counterclockwise.

I agree with the critique that the viable launch windows might be less frequent. Although SpaceX needs to send a lot of ships to Mars. Maybe some or most they send the regular way, some they send my way. If my way's better (probably not but maybe), then it would still be a marginal improvement.
 

Attachments

  • SpaceX shortcut.png
    SpaceX shortcut.png
    18.4 KB · Views: 20
IIRC ion propulsion has now been shown to w*rk (on a small scale.) It doesn't provide a huge "kick" but because it requires relatively little "fuel" (Xenon w*rks) and get it's energy from the sun, it can provide a small impulse for a long time, slowly building speed over half the trip - which can then be bled off by reversing the thrust over the later half of the trip.

Years ago I heard it postulated that ion propulsion could even provide enough propulsion to create artificial gravity on a long trip. Imagine constant propulsion at 32 feet per second per second. That would feel just like being on Terra Firma. On a trip to Mars, the ship might approach a million miles per hour before reversing the ship to slow down for entry into Mars atmosphere. Pretty cool concept.
 
Delta-V. It’s all about Delta-V.

Until we develop much more powerful methods of propulsion, we are stuck with long trips timed to planetary positions. And minimizing as much as possible the mass we are transporting.

We can’t really change the delta-v required to move a certain mass from point A to B. So, Musk is trying to bring down the cost of Delta-v by reusing rockets. So far so good, except for the SpaceX competitors.
 
It's interesting to think about the exploration of Mars. But a trip by living, breathing Human Beings is pretty far off in the future.

So much extra weight in the form of Food, Water, Oxygen and all those necessities of life. A journey like that would have to launch from our Moon to take advantage of the lower gravity. This might be feasible due to the recent discovery of ice in the Moon's polar caps. From the water, they can extract the Oxygen and Hydrogen.

There's also the Van Allen Radiation Belts. No Astronaut has spent any considerable time in the Outer Belt. Who knows what the long-term effect of exposure would be.

By the time we establish our Lunar Base and begin extracting Water, Oxygen and Hydrogen Peroxide for fuel, there will be AI robots available for such a long and hazardous journey.
 
Last edited:
I really believe we "must" go to Mars some day, but our robotic missions have been so successful that putting humans on Mars is not a critical priority. Yes, it needs to happen, but not for maybe 50 years or even 100 years.

If we can lean to more completely recycle during such a long trip, that could solve the weight problem to some extent. I really enjoyed the movie "The Martian." There were some seeds of recycling in it that just could w*rk. Imagine growing food from your own waste. All the sun energy you need would be available and we already know how to recycle water IIRC.
 
Imagine growing food from your own waste.
As I understand it the folks in the Democratic People's Republic of Korea (i.e., North Korea) have been doing that of necessity for decades. It works, sort of, but not very well.
 
By the time things get serious about going to Mars, newer more advanced technologies will likely emerge.

A 123,000 MPH Nuclear Rocket Could Reach Mars in Only One Month​

 
Would your slingshot around Venus bring the spaceship too close to the Sun ??
Well, they'd do it at night of course.

But I have a serious question:
I understand how a spacecraft would accelerate as it approaches the gravitational pull of another planet. But as it leaves that planet, why doesn't that same gravity slow the spacecraft down with an equal force?
 
Well, they'd do it at night of course.

But I have a serious question:
I understand how a spacecraft would accelerate as it approaches the gravitational pull of another planet. But as it leaves that planet, why doesn't that same gravity slow the spacecraft down with an equal force?

It would if the direction of travel were aligned with the centers of mass of the two objects. However, with the slingshot approach, the path of travel is offset to one side, so the gravitational force is on an angle which varies over time.

The slingshot part of the idea works; if it didn't they wouldn't have used it on Voyager. My reservations about my idea more relate to whether the "going backwards" aspect of it would work better.
 
Well, they'd do it at night of course.

But I have a serious question:
I understand how a spacecraft would accelerate as it approaches the gravitational pull of another planet. But as it leaves that planet, why doesn't that same gravity slow the spacecraft down with an equal force?
It does. The thrust of the rocket is more than the gravitational pull at the altitude of the launch site. As the rocket goes higher, the gravitational pull lessens and the rocket accelerates, because the thrust to gravitational pull ratio increases. Also, go listen to some of Elon discussions. He puts things in layman's terms about how rockets don't go straight up. They do their pitch and roll to accelerate on an orbital trajectory after launch - escape velocity happens as the rocket gradually gains altitude and speed going horizontal, not vertical. It's good stuff.
 
It does. The thrust of the rocket is more than the gravitational pull at the altitude of the launch site. As the rocket goes higher, the gravitational pull lessens and the rocket accelerates, because the thrust to gravitational pull ratio increases. Also, go listen to some of Elon discussions. He puts things in layman's terms about how rockets don't go straight up. They do their pitch and roll to accelerate on an orbital trajectory after launch - escape velocity happens as the rocket gradually gains altitude and speed going horizontal, not vertical. It's good stuff.
Speed is the issue. A rocket can go high up but will fall back down if it doesn’t have enough horizontal speed. Essentially the rocket has to go fast enough to always stay a bit out in front of the Earth so it never hits it as gravity pulls on it. Weird but it works.
 
As I understand it the folks in the Democratic People's Republic of Korea (i.e., North Korea) have been doing that of necessity for decades. It works, sort of, but not very well.
I guess for them "It's not rocket science."
 
Speed is the issue. A rocket can go high up but will fall back down if it doesn’t have enough horizontal speed. Essentially the rocket has to go fast enough to always stay a bit out in front of the Earth so it never hits it as gravity pulls on it. Weird but it works.
That's certainly the way we do it now. In fact in orbit, we haven't actually escaped the Earth. It's just that we don't fall back fast enough before the curvature of the Earth falls away from us. (Another way to say what you said.)

BUT theoretically, if you had a rocket that could go (pick a small number) 100 miles/hour (top speed) you could still escape Earth by going "straight up" but you would have to apply the propulsion for a very long time. Clearly it wouldn't w*rk in reality because you could not carry enough fuel to sustain the required thrust.

Full disclosure: I learned this concept back in 1963 when I watched the movie "The Mouse on the Moon."
 
It would if the direction of travel were aligned with the centers of mass of the two objects. However, with the slingshot approach, the path of travel is offset to one side, so the gravitational force is on an angle which varies over time.

The slingshot part of the idea works; if it didn't they wouldn't have used it on Voyager. My reservations about my idea more relate to whether the "going backwards" aspect of it would work better.
So you're saying that as the spacecraft pulls away from the other planet, that planet is moving away so it's gravitational pull is less and more short lived than when the spacecraft arrived?
 
Consider the frame of reference of the planet being used for the slingshot. In that frame the slingshot projectile does arrive and depart symmetrically. If the velocity of the planet is pointing in the direction you want the projectile to go then even with this symmetry the slingshot process adds velocity in the desired direction.
 
Just heard from my sister that Space-X is planning a launch and recovery of the Falcon Heavy this Sunday IF they can get approval...

She is staying down there and DW is hoping it happens so she can see it...

Sister has seen one before... one that they blew up soon after launch...
 
Well, they'd do it at night of course.

But I have a serious question:
I understand how a spacecraft would accelerate as it approaches the gravitational pull of another planet. But as it leaves that planet, why doesn't that same gravity slow the spacecraft down with an equal force?
It leaves at a shallower angle. FTR, I'm a trained rocket scientist but never worked in the field. Hohman transfers were the farthest I went. And the n-body problem was unsolved in my college days. I take my hat off to the later generations for what they have figured out.
 
Back
Top Bottom